1. Field of the Invention
The present invention is directed to a measuring system for physical parameters, such as for example, electrical current, voltage, temperature, strain, etc., and more particularly, to a measurement system which simultaneously makes a plurality of high speed measurements at different measurement sensitivities or ranges and outputs an optimal one of the measurements on a sample by sample basis.
2. Description of the Related Art
FIG. 1 is a block diagram of a conventional measuring system for measuring electrical current. In the current measuring system of FIG. 1, multiple current shunts RH and RL and amplifiers A1 and A2 are used to cover a dynamic range of a signal to be measured. In operation, one of the shunts RH or RL is selected by switches SH and SL. The amplifiers A1 and A2 are powered from power sources V1 and V2.
An output of amplifier A1 provides a low range measurement, ImonL, to an input 0 of a multiplexer 101 and an output of amplifier A2 provides a high range measurement, ImonH, to an input 1 of the multiplexer 101. A capacitor CL is connected in parallel with the resistor RL and provides damping during operation of the switches SL and SH. An output (Out) of the multiplexer 101 is provided as an input to an analog to digital (AND) converter 102. The A/D converter 102 provides a digitized output signal which is input to a logic gate array 103. The logic gate array 103 provides a control to the multiplexer 101 to select one of the input 0 and the input 1 of the multiplexer 101.
The current measurement system of FIG. 1 is inserted in a circuit to be measured so that a current I1 flows between in and Return as shown in FIG. 1. The current measurement system of FIG. 1 allows for a wide dynamic measurement range, high resolution and high accuracy measurements. However, measurements are not possible while the system is changing ranges and range changing is slow. Further, during a range change, the current I1 is disturbed. Therefore, the system must wait for the signal to settle before a measurement is taken. In a similar system (not shown), shunt resistors RL and RH are series connected and RL Is bypassed to make high range measurements. The series connected system has problems similar to the problems of the parallel shunt system shown in FIG. 1.
In another conventional current measurement system, a single shunt and multiple measurement amplifiers are employed as shown in FIG. 2. The single shunt measuring system of FIG. 2 is inserted in a circuit to be measured so that a current I1 flows between in and Return as shown in FIG. 2 and the current I1 flows through a resistor RM which is commonly connected with respective inputs of measurement amplifiers A3 and A4. The measurement amplifier A3 is a high gain amplifier and provides an output ImonL to the input 0 of the multiplexer 101. The measurement amplifier A4 is a low gain amplifier and provides an output ImonH to the input 1 of the multiplexer 101. The multiplexer 101, the A/D converter 102 and the logic gate array 103 operate in a similar manner as described with reference to FIG. 1.
In the measurement system shown in FIG. 2, as I1 increases, the high gain measurement amplifier A3 saturates and the system must smoothly transition to utilize feedback of the low gain amplifier A4. The measurement system of FIG. 2 allows for a smaller settling time when switching from a higher measurement range to a lower measurement range. However, the system of FIG. 2 does not provide continuous current measurements due to the saturation of the lower range measurements which are made by the high gain measurement amplifier A3. In addition, the system of FIG. 2 has an inherent disadvantage of providing poor resolution and accuracy of the measured signal at low current levels as the shunt RM must be sized to handle the entire dynamic range.